Limited modulation furnace and method for controlling the same

ABSTRACT

A limited modulation furnace and a method for controlling the limited modulation furnace using a floating control algorithm are disclosed. The method includes providing a furnace comprising a modulating furnace stage and at least one fixed heat output furnace stage, each fixed heat output furnace stage having an operational status of either on or off, passing a heating fluid through the furnace, measuring a temperature of the heating fluid exiting the furnace, determining an error based on a difference between the measured temperature and a set-point temperature, the set-point temperature being associated with a thermostat setting, determining whether the error is within a deadband range for the set-point temperature, and modifying at least one of the heat output of the modulating furnace stage or the operational status of at least one of the fixed heat output furnace stages in response to the error being outside the deadband range.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 60/658,835 filed Mar. 4, 2005, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to controlling heat output in a limited modulation furnace. More particularly, the present invention relates to controlling heat output in a limited modulation furnace using a floating control sequence.

BACKGROUND OF THE INVENTION

Fuel-fired heating furnace designs for use in heating a conditioned space have typically relied upon a single furnace firing rate. The furnace burner was either off or firing at a single maximum firing rate based on a control signal received from a thermostat or other control device. That is, these furnaces had a fixed heat output and operated in a simple “on/off” manner. While this conventional approach had the benefit of simplicity, it was undesirable from a comfort standpoint. More specifically, because of the inability of a fixed output furnace to precisely match the heating demand of a space to be heated, wide variations in conditioned space temperature was a common and undesirable occurrence.

In an attempt to overcome these undesirable temperature swings, modulating furnaces were developed. The amount of fuel introduced into the modulating furnace can be varied to produce a range of heat outputs, which can thereby reduce temperature swings in the conditioned space. A valve on the fuel line that delivers the fuel to the furnace is opened or closed depending on the amount of fuel required by the furnace. The amount of fuel required by the furnace is based on the particular operating conditions of the furnace and the heat output determined to be necessary. However, while a modulating furnace avoids the temperature swings of a simple fixed heat output furnace, a modulating furnace has its own drawbacks, including higher costs and a lower level of reliability resulting from the need for a modulating furnace large enough to meet the capacity required to span a full range of heating needs for a conditioned space.

Accordingly, what is needed is a furnace that avoids temperature swings in the conditioned space while providing for reduced costs and higher reliability.

SUMMARY OF THE INVENTION

A limited modulation furnace is provided. The limited modulation furnace includes one or more fixed heat output stages and a modulating stage. By using one or more fixed heat output stages in combination with a modulating stage having a smaller range of heat outputs, the limited modulation furnace can provide heat output levels to meet a full range of desired conditions, similar to that provided by a large single modulating furnace.

A method for controlling a limited modulation furnace is disclosed. The method includes the steps of providing a furnace comprising a modulating furnace stage and at least one fixed heat output furnace stage, each fixed heat output furnace stage having an operational status of either on or off, passing a heating fluid through the furnace, measuring a temperature of the heating fluid exiting the furnace, determining an error based on a difference between the measured temperature and a set-point temperature, the set-point temperature being associated with a thermostat setting, determining whether the error is within a deadband range for the set-point temperature, and modifying at least one of the heat output of the modulating furnace stage or the operational status of at least one of the fixed heat output furnace stages in response to the error being outside the deadband range.

According to another exemplary embodiment of the invention, a method for controlling a limited modulation furnace includes the steps of providing a furnace comprising a modulating furnace stage and at least two fixed heat output furnace stages, each fixed heat output furnace stage having an operational status of either on or off, passing a heating fluid through the furnace, measuring a temperature of the heating fluid exiting the furnace, determining an error based on a difference between the measured temperature and a set-point temperature, the set-point temperature being associated with a thermostat setting, determining whether the error is within a deadband range for the set-point temperature, and modifying at least one of the heat output of the modulating furnace stage or the operational status of at least one of the fixed heat output furnace stages in response to the error being outside the deadband range, the modifying of the heat output of the modulating furnace stage at a rate determined by a floating control algorithm and wherein the floating control algorithm determines the rate at least in part on a magnitude of the determined error using a rate compensation algorithm.

A limited modulation furnace is also disclosed. The limited modulation furnace comprises a fluid path having an inlet and an outlet for a heating fluid to pass therethrough, a modulating furnace stage having a variable output capacity when in operation, the modulating furnace stage positioned to heat the heating fluid in the fluid path, at least one fixed heat output furnace stage, each fixed heat output furnace stage having a single output capacity when in operation, each fixed heat output furnace stage positioned to heat the heating fluid in the fluid path, a temperature sensor positioned to detect a temperature of the heating fluid in the fluid path, wherein the temperature sensor is in electronic communication with a controller, and a modulating valve intermediate the modulating furnace stage and a fuel source, wherein the modulating valve is in electronic communication with the controller and wherein, in response to a rate determined by the controller using a floating control algorithm, the modulating valve modifies the volume of fuel passing to the modulating furnace stage thereby modifying the output capacity of the modulating furnace stage.

One advantage of the present invention is a modulated heat output without relying solely on a single stage modulating furnace.

Another advantage of the present invention is that the heat output can be precisely controlled without additional control sequences that are required when using a PI control with a limited modulation furnace.

Another advantage of the present invention is that the proportional gain of the system does not have to be known to execute proper system control.

Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example only, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a limited modulation furnace according to an embodiment of the invention.

FIG. 2 illustrates a control sequence and staging logic for a limited modulation furnace as shown in FIG. 1.

FIG. 3 illustrates a floating control algorithm for use with a limited modulation furnace according to an embodiment of the invention.

FIG. 4 is a graph which illustrates the change in the limited modulation furnace ramp rate as a function of the error in the limited modulation furnace.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a limited modulation furnace 10. A fluid to be heated flows through a fluid path 12 where it passes over and/or through a series of furnace stages. The heated fluid is then distributed in a conventional manner to provide a desired level of heating to a conditioned space. The fluid can be any suitable fluid for providing heating to a conditioned space, such as water or air.

The limited modulation furnace 10 has a modulating furnace stage 20, designated as F_(MOD) and at least one fixed output, or on/off, type furnace stage that provide either a maximum heat output or no heat output. Preferably, the limited modulation furnace 10 includes at least two fixed output furnace stages. The furnace stages may be arranged in any order in the fluid path 12. As illustrated in FIG. 1, the limited modulation furnace 10 has three fixed output furnace stages 22, 23, 24 designated as F₁, F₂, and F₃ in addition to the modulating furnace stage 20. While FIG. 1 shows three fixed output stages, it is to be understood that any suitable number of fixed stages can be used. The fixed output furnace stages 22, 23, 24 preferably all have the same output capacity, but may have different output capacities.

The modulating furnace stage 20 has a heat output that can be varied incrementally from no output increasing in pre-determined increments up to a maximum heat output which is the maximum output capacity of the modulating furnace stage 20. Preferably, the maximum output capacity of the modulating furnace stage 20 is equal to the capacity of any one of the fixed output furnace stages 22, 23, or 24. That is, the maximum output capacity of F_(MOD) is preferably equal to that of F₁ or F₂ or F₃. As previously stated, it also preferable for the capacity of the fixed output furnace stages 22, 23, 24 are equal to one another. In this preferred embodiment, (i.e., F_(MOD)=F₁=F₂=F₃) as the number of fixed output stages is increased, the capacity of individual fixed output stages and, thus, the necessary maximum capacity of the modulating furnace stage 20 to maintain the same overall capacity of the limited modulation furnace 10, may be lowered. Because of the high costs of modulating furnace stages that have a large maximum capacity to achieve a broad range of heat outputs, having a larger number of smaller capacity fixed output furnace stages and a correspondingly smaller modulating furnace stage may actually decrease the overall system cost compared to a limited modulation furnace of the same total capacity but having a fewer number of fixed output furnace stages and thus requiring a larger modulating furnace stage.

Each furnace stage, whether modulating or fixed, is fired by the combustion of fuel in a burner (not shown) associated with each furnace stage. While the fuel is typically natural gas, other fuels could also be used in conjunction with an appropriately selected burner configuration within each furnace stage. Fuel from a common fuel source (not shown) or multiple fuel sources is introduced into the burner of each of the fixed output furnace stages 22, 23, 24 via a fuel line 30. The fuel line 30 also introduces fuel into the modulating furnace stage 20.

A modulating valve 50 located on the portion of the fuel line 30 entering the modulating furnace stage 20 is opened or closed to respectively increase or decrease the amount of fuel introduced into the modulating furnace stage 20. The position of the modulating valve 50 is controlled in response to a signal received from a controller 52, such as a central processing unit (CPU), a microprocessor or other suitable control device capable of executing a control algorithm. Likewise, each fixed output furnace stage 22, 23, 24 also includes an on/off valve 32, 33, 34, such as a gate valve, that can be in either one of an open or a closed position to permit or prohibit fuel from the fuel line 30 into the respective fixed furnace stage associated with a particular on/off valve, and which on/off valve is in communication with, and controlled by, the controller 52.

A temperature sensor 54 placed in the fluid path 12 measures the temperature of the fluid leaving the last stage of the limited modulation furnace 10. The temperature sensor 54 is in electronic communication with the controller 52. The controller 52 then compares the measured temperature of the heated fluid with a set-point temperature that is associated with or based on a temperature selected to achieve the desired temperature in the space to be conditioned, typically using a thermostat to select a thermostat setting. It should be appreciated that while the thermostat setting may establish the set-point, it need not itself be the set-point.

A proportional-integral (PI) control can be effectively used to control a single stage modulating furnace and maintain or adjust to the desired heat output level. The proportional gain in the PI control is proportional to the temperature rise, which gain is usually known based on the change in temperature. However, in a limited modulation furnace having at least one fixed heat output stage, temperature rise on the furnace is often unknown, especially if one of the fixed heat output stages becomes disabled or fails. This makes the use of a PI control to control the limited modulation furnace impractical absent the addition of other more complex controls. While a proportional gain for the PI control may be arbitrarily assigned or assigned for a worst case scenario, this results in an unacceptably slow responding system, except where the worst case scenario actually occurs.

The control algorithm executed by the controller 52 is a floating control algorithm, in which the rate at which the overall heat output of the limited modulation furnace 10 changes, or “floats,” is based on the disparity between the desired and actual temperatures of the conditioned space. The floating control algorithm provided, along with selection of an appropriate deadband, does not require the proportional gain of the system to be known. The floating control algorithm determines an error based on a temperature differential and then modifies the heat output of the modulating furnace stage and/or changes the current operational status of at least one of the fixed heat output furnace stages from on to off or vice versa in response to the determined error. The rate at which the heat output of the modulating furnace stage is modified is based on a floating scale which correlates to the magnitude of the determined error. As previously discussed, although PI controls have typically been used in conventional single stage modulating furnaces, the use of a PI control in a limited modulation furnace having at least one fixed output furnace stage in addition to a smaller modulating furnace stage is impractical due to an unknown temperature rise on the system and correspondingly an unknown proportional gain. Using a floating control avoids the need to know or arbitrarily assign a proportional gain and only requires knowledge of the integral gain (which is proportional to the system response) to establish a rate of change for the furnace's heat output that insures good control without oscillation.

FIG. 2 illustrates a control sequence and staging logic for the limited modulation furnace 10 as shown in FIG. 1. At the lowest desired level of heat output in response to a low demand for heat, each of the fixed output stages 22, 23, 24 is in the “off” setting, while the modulating furnace stage 20 exhibits a linear increase in output in response to an increased demand for heat as the percentage of fuel to that stage is increased by opening the modulating valve 50. When the level of heat output of the modulating furnace stage 20 reaches its maximum heat output, the operational status of a first fixed output stage 22 is modified such that the fixed output stage 22 is switched “on.” The other two fixed output stages 23, 24 remain off. The modulating furnace stage 20 is decreased to its minimum level; as more heat output is required in response to an increasing demand for heat, the output of the modulating furnace 20 is again gradually increased. Once the total heat output of the modulating furnace stage 20 is again at its maximum level, the second fixed output stage 23 is switched “on” and the modulating furnace stage 20 is again decreased. This process is repeated until the overall desired heat output is reached or until the total heat output reaches the maximum output capacity of the limited modulation furnace 10.

In a corresponding manner, if the heat output needs to be decreased to reach the desired temperature, the process is reversed, with fixed heat output furnace stages being turned “off” as the modulating furnace stage 20 is decreased.

FIG. 3 shows a flowchart of the floating control algorithm executed by the controller 52 to change the heat output of the limited modulation furnace 10. The algorithm begins by determining and evaluating an error and thus whether any modification to the heat output is required. The “error” is defined as a process variable minus a corresponding set-point for the variable, e.g. the temperature determined by the temperature sensor 54 minus a set-point temperature corresponding to the temperature at which the thermostat is set. The “deadband” is the temperature range about the set-point at which no change in the heat output will occur. The deadband is typically selected to be about 3° F., but it may be greater or lower depending on the capacity of the system, the size of the space to be conditioned, or other considerations as are known to those of skill in the art. For example, if the set-point is 75° F., the deadband may be selected to be 3° F., so that no change is made in the furnace's heat output if the temperature is within 73.5° F. and 76.5° F.

At s10, the error is evaluated to determine whether it is greater than or equal to one half of the deadband. If the error at s10 is determined to be greater than or equal to one half the deadband, it is determined that the heat output is greater than needed to achieve the desired temperature and the algorithm proceeds through the Ramp Down portion of the algorithm to decrease the heat output. In the Ramp Down portion, the algorithm evaluates whether the modulating furnace stage 20 is at its minimum output level at s20. If so, then the process passes to s22, in which a floating variable, designated N, is evaluated. The floating variable is used to designate which fixed output furnace stage is being considered with respect to the current loop of the algorithm. If N=0, then only the modulating furnace stage 20 is operating and the limited modulation furnace 10 is producing as low a heat output as possible (which may include no heat output, where the modulating valve is fully closed) and the process returns to the beginning and the error is again evaluated at s10. If N is not equal to 0, then the fixed output stage corresponding to N is turned off at s24. After stage N is turned off, N is set to N−1 at s26. To compensate for the loss of heat for the Nth stage that was turned off at s24, the modulating furnace stage 20 is then set to its maximum output at s28 to either maintain that level of heat or to further decrease the heat. After s28, the process passes to s10 for further evaluation. It will be appreciated that although steps s26 and s28 are shown in a particular order in FIG. 3, this order may be reversed or these steps may occur simultaneously.

If the error measured at s10 is greater than one half the deadband, but the modulating furnace stage output is not at its minimum level when evaluated at s20, the process instead passes to s25, in which the output of the modulating furnace stage is decreased at a rate determined by a rate compensation algorithm.

During startup of the limited modulation furnace 10 or when a sudden change in set-point occurs, a sudden error results between the output temperature of the fluid leaving the furnace 10 and the set-point. Although the proportional term of a PI control would compensate for this sudden shift, the floating control algorithm does not have a proportional term and cannot quickly compensate for these sudden changes in error based on a single ramp rate. To overcome this deficiency, a rate compensation algorithm is provided as a subroutine of the floating control algorithm that adjusts the ramp rate proportional with the error between the process variable and the set-point. In other words, if the error is large, the ramp rate is also large, while if the error is small, the ramp rate will also be small. In this manner, the rate at which the heat output is increased or decreased “floats” based on the error.

An exemplary error versus ramp rate correlation diagram from which the rate compensation algorithm determines the ramp rate is shown in FIG. 4. The ramp rate increases in a linear fashion as the magnitude of error increases, however it will be appreciated that the ramp rate could vary in a non-linear fashion as well. Based on the correlation chart, the rate compensation algorithm determines the proper rate at which the heat output of the modulating furnace stage 20 decreases toward its minimum output.

The rate compensation determination is a function of the cycle time of controller 52 and the opening time, i.e., the amount of time to open the modulating valve from its minimum to maximum output. The cycle time is primarily determined by the time needed to analyze the temperature measured by the temperature sensor 54, calculate an appropriate response, and send a corresponding signal to the appropriate valves to result in a change in heat output. Typical cycle times are about 1 second, preferably less, depending on the size and speed of the microprocessor components used.

Where the modulating valve 52 opens over time in a linear fashion, the opening time may be described as follows: $\begin{matrix} {{OpeningTime} = {{\left( \frac{t_{\max} - t_{\min}}{T_{\max} - T_{\min}} \right)*{error}} + b}} & (1) \end{matrix}$

where

t_(max)=a predetermined fastest time (in seconds) at which the modulating valve is adjusted between its maximum and minimum positions,

t_(min)=a predetermined slowest time (in seconds) at which the modulating valve is adjusted between its maximum and minimum positions,

T_(max)=a maximum error, in terms of temperature (in ° F.), at or above which the modulating valve is assigned to open at t_(max),

T_(min)=a minimum error, in terms of temperature (in ° F.), at or below which the modulating valve is assigned to open at t_(min),

error=the absolute difference between the setpoint and the actual temperature (in ° F.), and

b=the y-intercept, corresponding to the opening time (in seconds) when the error is zero.

Once the opening time has been determined, the rate of change, i.e., the percentage change by which the modulating valve is adjusted open or closed in a single cycle, can be determined by using the following equation: $\begin{matrix} {{Rate} = {\left( \frac{CycleTime}{OpeningTime} \right)*100\%}} & (2) \end{matrix}$

The rate compensation algorithm described above is further illustrated by way of the following non-limiting example. In a particular limited modulation furnace, the fastest opening rate for the modulating valve 52 (t_(max)) may be predetermined as 120 seconds, which time may be selected to correspond to an error that is equal or greater than 15° F. (T_(max)) from the setpoint. Conversely the slowest opening rate (t_(min)) may be predetermined to be 360 seconds, selected to correspond with an error measured to be equal or less than 1° F. (T_(min)). Under these circumstances, it has been determined that b is equal to 377 seconds. Thus, under these circumstances, if the error was measured to be 7° F., the rate of change is calculated as follows: ${OpeningTime} = {{{\left( \frac{120 - 360}{15 - 1} \right)*7^{\circ}F} + 377} = {257\quad{Seconds}}}$ ${Rate} = {{\left( \frac{1}{257} \right)*100\%} = {0.39\%\quad{Per}\quad{Cycle}}}$

Thus, with respect to FIG. 3, which illustrates a single cycle, in this example, the modulating furnace stage would be decreased at a rate of 0.39% until the controller passes through its next cycle, any error present is calculated, and the floating control algrorithm is called to pass through another iteration.

Returning again to FIG. 3, from s25, the process then returns to s10 where the error is again evaluated to determine whether the error is now within the deadband, or if not, whether the heat output of the modulating furnace stage 20 should continue to be decreased toward its minimum output.

When the error is not greater than or equal to one half the deadband, the process passes to s30, where the error is further evaluated to determine whether it is less than or equal to minus one half the deadband. In other words, if the temperature of the fluid leaving the limited modulation furnace 10 is not too warm, it is then evaluated as to whether it is too cold. If not, then the algorithm has determined that the current heat output is correct for the desired temperature and the process returns to s10 for further evaluation as the temperature over time either decreases or increases to a point outside the deadband. While FIG. 3 shows a particular order for error evaluation, it will be appreciated that the error could be determined in an opposite fashion, such as determining whether the error requires a ramp up of heat output before determining whether the error requires a ramp down in heat output.

If the error is less than minus one half the deadband, then the temperature is below the acceptable range and the total heat output of the furnace 10 must be increased to arrive at the desired temperature. The process then passes from s30 to the Ramp Up portion of the floating control algorithm. At s40, the algorithm evaluates whether the modulating furnace stage 20 is operating at maximum output. If not, the process passes to s45 and the output of the modulating furnace stage 20 is increased at a rate determined by the rate compensation algorithm in the same manner as the rate of decrease was determined in the Ramp Down portion of the floating control algorithm. It will be appreciated that the error on the x-axis of the graph of FIG. 4 represents the absolute value of the error, such that the same ramp rate applies to a particular magnitude of error, regardless of whether the actual temperature is above or below the set-point, and thus whether the heat output is being ramped up or down. It will further be appreciated, however, that separate rate compensation algorithms could be used depending on whether heat output is to be increased or decreased.

If the modulating furnace stage 20 is operating at its maximum output, the process passes to s42 in which N is evaluated to determine whether it is equal to the total number of fixed output stages in the limited modulation furnace 10. If so, then the furnace 10 is at maximum capacity and the process returns to s10 for further evaluation of the error as the temperature in the conditioned space increases. If N does not equal the total number of stages, then the process passes to s44 and N is incrementally increased to equal N+1. Thereafter, the fixed output stage N is switched to an “on” position at s46. Finally, as a result of turning on a fixed output stage, the modulating furnace stage 20 is returned to its minimum output, following which the process returns to s10 for further evaluation on the need for any further adjustments in the heat output. As in the Ramp Down portion of the floating control algorithm, it will again be appreciated that steps s46 and s48 can be performed in any order or simultaneously.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A method for controlling a limited modulation furnace, the method comprising the steps of: providing a furnace comprising a modulating furnace stage and at least one fixed heat output furnace stage, each fixed heat output furnace stage having an operational status of either on or off; passing a heating fluid through the furnace; measuring a temperature of the heating fluid exiting the furnace; determining an error based on a difference between the measured temperature and a set-point temperature, the set-point temperature being associated with a thermostat setting; determining whether the error is within a deadband range for the set-point temperature; and modifying at least one of the heat output of the modulating furnace stage or the operational status of at least one of the fixed heat output furnace stages in response to the error being outside the deadband range.
 2. The method of claim 1, wherein modifying the heat output of the modulating furnace stage occurs at a rate determined by a floating control algorithm.
 3. The method of claim 2, wherein the rate at which the heat output is modified occurs in a linear fashion based on a magnitude of the error.
 4. The method of claim 2, wherein the rate at which the heat output is modified occurs in a non-linear fashion based on a magnitude of the error.
 5. The method of claim 1, wherein the step of providing a furnace further comprises providing at least two fixed heat output furnace stages having equal maximum output capacities.
 6. The method of claim 5, wherein the step of providing a furnace further comprises providing a modulating furnace stage having a maximum output capacity equal to a maximum heat output capacity of a fixed heat output furnace stage.
 7. The method of claim 2, wherein the floating control algorithm includes a subroutine including a rate compensation algorithm.
 8. The method of claim 7, wherein the rate compensation algorithm for modifying the heat output is used to both increase heat output and decrease heat output.
 9. The method of claim 1 wherein the modifying of the heat output of the modulating furnace stage is performed by a modulating valve controlled by a processing unit in electronic communication with a temperature sensing unit in contact with the heating fluid.
 10. A method for controlling a limited modulation furnace, the method comprising the steps of: providing a furnace comprising a modulating furnace stage and at least two fixed heat output furnace stages, each fixed heat output furnace stage having an operational status of either on or off; passing a heating fluid through the furnace; measuring a temperature of the heating fluid exiting the furnace; determining an error based on a difference between the measured temperature and a set-point temperature, the set-point temperature being associated with a thermostat setting; determining whether the error is within a deadband range for the set-point temperature; and modifying at least one of the heat output of the modulating furnace stage or the operational status of at least one of the fixed heat output furnace stages in response to the error being outside the deadband range, the modifying of the heat output of the modulating furnace stage at a rate determined by a floating control algorithm and wherein the floating control algorithm determines the rate at least in part on a magnitude of the determined error using a rate compensation algorithm.
 11. A limited modulation furnace comprising: a fluid path having an inlet and an outlet for a heating fluid to pass therethrough; a modulating furnace stage having a variable output capacity when in operation, the modulating furnace stage positioned to heat the heating fluid in the fluid path; at least one fixed heat output furnace stage, each fixed heat output furnace stage having a single output capacity when in operation, each fixed heat output furnace stage positioned to heat the heating fluid in the fluid path; a temperature sensor positioned to detect a temperature of the heating fluid in the fluid path; a controller in electronic communication with the temperature sensor; and a modulating valve intermediate the modulating furnace stage and a fuel source, wherein the modulating valve is in electronic communication with the controller and wherein, in response to a rate determined by the controller using a floating control algorithm, the modulating valve is configured to modify the volume of fuel passing to the modulating furnace stage thereby modifying the output capacity of the modulating furnace stage.
 12. The limited modulation furnace of claim 11 comprising at least two fixed heat output furnace stages.
 13. The limited modulation furnace of claim 12, wherein the single output capacity of each fixed heat output furnace stage is equal to the single output capacity of every other fixed heat output furnace stage in the limited modulation furnace.
 14. The limited modulation furnace of claim 13, wherein the modulating furnace stage has a maximum output capacity equal to the single output capacity of each fixed heat output furnace stage.
 15. The limited modulation furnace of claim 11, wherein the limited modulation furnace comprises at least three fixed heat output furnace stages. 